of course! Feel free to correct, edit, complain
I'm looking at presenting the information based on how people may search for it...i.e. "Can I read my Kindle books on an iPad?"
DRM Formats, Vendor Specific:
iBooks (Fairplay ePub)
iBooks
Kindle (various formats)
Kindle App, by Amazon (iPad and iPhone apps available, no sideloading)
Barnes and Noble (Adobe DRM-ADEPT Variant- PDF and ePub)
Barnes and Noble eReader Apps, by Barnes and Noble (iPad and iPhone apps available, no sideloading)
Borders Books (Adobe ADEPT(?) PDF and ePub)
Borders eBooks for iPhone and iPad
Kobo Books (Adobe ADEPT(?) PDF and ePub)
Kobo Books by Kobo (also Borders)
Sony Books (Adobe ADEPT(?) ePub)
Txtr, by Txtr (iPhone app available)
DRM Formats, General:
Adobe ADEPT (PDF and ePub)
Kobo, by Kobo (iPad and iPhone apps available, no sideloading)
Txtr, by Txtr (iPhone app available)
Non-DRM Formats:
ePub
iBooks, by Apple (iPad app available now, iPhone app 6/21)
Stanza App, by Lexcycle (iPad and iPhone apps available-thru iTunes)
IBIS Reader, by Threepress (HTML5 based, no app required)
PDF
Stanza, by Lexcycle (iPad and iPhone apps available-thru iTunes)
Cloudreaders by Cloudreaders
GoodReader by Good-i-Ware
iBooks, by Apple (for both iPad and iPhone on 6/21)
Mobi
Stanza, by Lexcycle (iPad and iPhone apps available-thru Stanza Desktop)
PDB
iSilo by iSilo (iPad and iPhone apps available)
Stanza, by Lexcycle (iPad and iPhone apps available-thru Stanza Desktop)
LIT
Stanza, by Lexcycle (iPad and iPhone apps available-thru Stanza Desktop)
CHM
Bookshelf by HPC Technologies (iPad and iPhone apps available)
Other:
Comics (CBR, CBZ)
Stanza, by Lexcycle (iPad and iPhone apps available-thru iTunes)
Cloudreaders by Cloudreaders
Comic Zeal by bitolithic.
Bookman, by Bookman Lab
Marvel Comics by Marvel
Unsupported:
Sony (LRF, LRX) by Sony. (deprecated)
here as well , thanks, nikkie!